I'm not FE myself, however I always like to consider what the best argument might be for the other side, and sometimes you can force yourself to consider new things this way.
So something that bothers me here is that not all sunsets look like this, and neither flat not round Earth dictates that the atmosphere and the clouds in it must hug the ground with even thickness at all times. It doesn't make sense to me then that this picture proves that the ground must be at a curve. These clouds could easily be set at their own arbitrary angles, independent from the ground, due to areas of high/low pressure.
Go search for underwater photography, like particularly viewing a wave with surf from beneath, and you'll see what I'm getting at here. Some of these photos look strikingly similar to weather systems you might see in the sky. It's not difficult to imagine how a sunset might light the surf from beneath if the surface was gently sloped by a long wave.